I may have over thought the shoulder straps. I didn't do a Hong Kong finish on the edges where the straps attach to the body because I didn't want any seam allowance visible when the stays are seen from a front angle. So I hid the seam allowances inside in a probably overdone way.
I pinned the Toile strap to the Toile body only, making sure not to catch any denim.
After sewing the Toile pieces together I sewed the denim strap, but I set it 1/8 inch from the body piece so the seam allowances wouldn't fall at the same spot.
I pressed the Toile allowance up, the denim allowance down, then graded the seam allowances.
Then from the front I stitched in the ditch right along the seam line.
The straps have just the one seam across the front,
and if I've assembled them the way I think I have there should be a line of stitching 1/8 from the seam on the other side securing the straps to each other.